Fishing line! Seriously, anchor the tree to the wall, just tie fishing line around the trunk then run a piece over to the wall, where you can anchor in a discreet spot. Do at least two, three if you can, and that will make the tree much harder to tip over. It's either that or be prepared to be vigilant with a spray bottle when you are home, and keep him out of the room when you're not home! It's kind of easier to make the tree more stable use unbreakable ornaments, and let them explore until they are bored with it! No tinsel, though - you probably already know that can wreak havoc with kitty tummies, so it's best to avoid temptation!